Since lithium ion secondary batteries that bind and give off lithium ions have advantages such as high energy densities, high operating voltages, and so forth over nickel cadmium (Ni—Cd) batteries and nickel metal hydride (Ni-MH) batteries of the same capacities, they have been widely used for information processing devices and communication devices such as personal computers and mobile phones that require miniaturization and lightweightness.
Moreover, in recent years, lithium ion secondary batteries have been assessed to be usable as power supplies for electric bicycles, hybrid automobiles, and so forth and also they are being introduced as batteries that store electric power generated by renewable power supplies such as solar batteries to realize a low-carbon society that solves global warming problems.
To enable the widespread use of lithium ion secondary batteries for electric power storage and as a high capacity power supply for electric automobiles, it is necessary to reduce the maintenance cost as well as manufacturing cost, thereby to prolong their product life.
Although it is thought that the product life of lithium ion secondary batteries can be extended by re-examining the materials that comprise them and the structure of the batteries, there is a method that can reduce the shortening of their product life cycles that is caused by inappropriate usage of the battery and so forth. For example, Patent Document 1 and Patent Document 2 propose techniques that reduce the shortening of the life cycles of lithium ion secondary batteries by controlling charging and discharging of these batteries.
Patent Document 1 describes that charging and discharging of a lithium ion secondary battery are controlled such that the number of lithium ions that migrate between a positive electrode material and a negative electrode active material, when the lithium ion secondary battery is charged or discharged, is 95% or less of the number of lithium ions that migrate in the reverse direction.
Patent Document 2 describes that charging and discharging of a lithium ion secondary battery are controlled such that the end-of-discharge voltage, when the lithium ion secondary battery is discharged, ranges from 3.2 to 3.1 V and such that the upper limit voltage when the lithium ion secondary battery is charged ranges from 4.0 to 4.5 V.
As positive electrode materials (positive electrode active materials) of lithium ion secondary batteries, compositions using lithium cobalt oxide, lithium manganese oxide, and lithium nickel oxide are known. As negative electrode materials (negative electrode active materials), compositions using graphites and cokes are known.
The applicant of the present patent application discovered that when a manganese lithium ion secondary battery having lithium manganese oxide that is used for the positive electrode material of various types of lithium ion secondary batteries is stored at a particular SOC (State of Charge), the battery performance quickly deteriorates (the battery capacity deteriorates).
In this context, SOC represents the ratio of the capacity of the lithium ion secondary battery to the amount of electric charge. The particular SOC in which the battery performance quickly deteriorates is less than the maximum SOC that is the charging limit point and greater than the minimum SOC that is the discharging limit point, for example SOC=40%. In addition, “store” in the specification of the present patent application denotes that a lithium ion secondary battery is kept in the state of a particular voltage of the SOC.
The phenomenon in which the battery performance deteriorates in the particular SOC is not significantly related to a case in which the lithium ion secondary battery is stored in the fully charged state, for example, when it is used for a UPS (Uninterruptable Power Supply).
However, in an application where a lithium ion secondary battery is stored at any SOC between the maximum SOC and the minimum SOC, for example, in an application where electric power generated by the above-described renewable power supply is stored, the lithium ion secondary battery can be understood as being kept in the above-described particular SOC. In such a case, the battery performance of the lithium ion secondary battery will quickly deteriorate.